This invention relates to a power supply for an electrical load which requires a modulated input signal for proper operation, and more particularly to a ballast for a high-intensity gas discharge lamp.
Certain types of electrical loads operate more efficiently or effectively when driven by an input current signal that is both alternating and modulated in some manner. One example of such an electrical load is a high-intensity gas discharge (HID) lamp, of the type used indoors or outdoors for general illumination of parking lots and buildings.
As disclosed in U.S. Pat. No. 6,184,633 B1, the operational stability and uniformity of light generated by such HID lamps can be improved by driving the lamps with a swept frequency input signal that is either amplitude modulated, or sequentially alternated with a periodic change in the range of the swept frequency. In particular, an input signal is disclosed that includes a current frequency sweep from 45 to 55 kHz within a sweep time of about 10 milliseconds, in combination with either: a constant amplitude modulation frequency of about 24.5 Hz and a modulation index of 0.24; a periodic sequential change in sweep frequency to about 12.3 kHZ. In the ""633 B1 patent, the swept frequency and amplitude modulating signals are generated separately and combined in a summing junction to produce the signal actually fed to the HID lamp. The modulation frequency is predetermined and maintained as a constant.
U.S. patent application Ser. No. 09/620,357 now U.S. Pat. No. 6,400,100, commonly assigned to the original assignee of the present application, provides additional methods and apparatus for determining the optimum modulation frequency and/or sequential operating frequency, through sequential and periodic measurement of maximum operating current and/or voltage delivered to the HID lamp at various amplitude modulating frequencies within a predetermined range of frequencies. Once the appropriate values are determined, the ""357 application utilizes the methods and apparatus disclosed in the ""633 B1 patent for producing the input waveform.
Although the teachings of the above referenced patent and application provide considerably improved performance with electrical loads, such as HID lamps, additional improvement is desirable. The above referenced patent relies on predetermined operating parameters believed to provide optimal performance. The above referenced patent application improves on this by utilizing feedback measurements of lamp current and voltage to adjust operating parameters for optimal performance. Variations in performance between HID lamps, and installation specific considerations, such as horizontal vs. vertical installation positions of the lamp, may introduce variables that are not addressed by either the above referenced patent or application. It is also desirable that a power supply and/or ballast be adaptable for providing optimized performance with more than one type of load (HID lamp type or size).
What is needed, therefore, is an improved power supply and/or ballast.
Our invention provides such an improved power supply and/or ballast through recognition that it is really the power spectrum delivered to the electrical load that is important in optimizing performance. Our invention monitors the current and voltage signal driving the load, and calculates a representative portion of the power spectrum being delivered to the load. The calculated portion of the delivered power spectrum is compared to a desired power spectrum, and the input signal to the load is modified in such a manner that the delivered power spectrum will more closely match the desired power spectrum. The input current and voltage, and the frequency at the load are thus not controlled directly, as was the case in the prior art, but are an indirect result of applying the modified input power spectrum to the load. By utilizing the input power spectrum in this manner, our invention provides a power supply and/or ballast that achieves improved control and performance when used with electrical loads such as HID lamps.
One form of our invention provides a method for operating a power source adapted to supply an input signal having an alternating current and voltage to an electrical load in a manner providing a desired power spectrum to the electrical load. The method includes calculating a portion of the power spectrum being delivered to the electrical load by the input signal, and modifying the input signal in such a manner that the delivered power spectrum more closely matched the desired power spectrum. The step of calculating a portion of the delivered power spectrum may include monitoring and integrating the alternating current and voltage of the input signal. The method may further include storing a representation of the desired power spectrum in the power source.
In some embodiments of our invention, the power source includes a pulse width modulated signal generator. The step of modifying the input signal includes controlling the pulse width modulated signal generator to produce the desired power spectrum. In embodiments where it is desired that the input signal be an amplitude modulated swept frequency signal, having a number of parameters including an amplitude, a sweeping frequency ranging between a first lower frequency and a second higher frequency value in a specified period of time, an adjustable modulating frequency, and an adjustable modulation index, the step of modifying the input signal includes controlling the pulse width modulated signal generator to adjust one or more of these parameters. Our method may also include additional steps for independently selecting the value of one or more of the parameters, on the basis of a sweep of current or voltage delivered to the electrical load at various modulating or sweeping frequencies.
Our invention may also take the form of an apparatus or code on a computer readable medium for performing the method of our invention.
In one form of our invention, an apparatus is provided for operating a high intensity discharge lamp (HID lamp) in a manner producing a desired power spectrum for driving the lamp. The apparatus includes a pulse width modulator having a first input adapted for connection to a source of electrical power, a second input for receiving a control signal, and an output adapted for coupling to the HID lamp. The pulse width modulator is configured to provide an amplitude modulated swept frequency signal to the lamp, with the signal having an alternating current and voltage and generating a delivered power spectrum at the lamp, when the apparatus is connected between a source of electrical power and the lamp. The apparatus also includes a controller having an input operatively coupled to the output of the pulse width modulator, for receiving the alternating current and voltage signal, and an output operatively connected to the second input of the pulse width modulator for delivering a control signal to the pulse width modulator. The controller is configured for calculating a portion of the delivered power spectrum, and for providing a control signal to the pulse width modulator for modifying the input signal in such a manner that the delivered power spectrum more closely matches the desired power spectrum.
The controller of the apparatus may include a processor for monitoring and integrating the alternating current and voltage input signal to compute a portion of the delivered power spectrum. The controller may also include means for storing a representation of the desired power spectrum.
The apparatus may also be implemented in either an analog or a digital pulse width modulated form, using either symmetric or asymmetric PWM control.
The foregoing and other features and advantages of our invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of our invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.